Broken activity feed when image or line break used in comment

Summary

When someone posts a comment containing a line break, the produced Atom feed will be invalid.

Steps to reproduce

1. Send a comment to containing either foo\
bar or foo<br>bar to an issue.
2. Visit project’s Activities Atom feed.

Example Project

Check https://gitlab.com/jtojnar/extensions.atom for example, which is broken by the first comment jtojnar/extensions#1.

What is the current bug behavior?

The feed will contain a literal <br> element, breaking the Atom feed.

What is the expected correct behavior?

The feed should contain a valid XML code.

Output of checks

This bug happens on GitLab.com

Possible fixes

See https://validator.w3.org/feed/docs/atom.html#text:

• The input needs to be sanitized to be valid XML if we want to use type=xhtml,
• or switch to type=html and
  • either escape the HTML code,
  • or wrap it with CDATA.